1. Field of the Invention
The present invention relates to the detection of gas and flames produced during a combustion process. More particularly, the present invention relates to the detection of gas and oil flames with emission characteristics in the ultraviolet light spectrum from 1,850-2,600 angstroms.
2. Description of the Prior Art
It is known to monitor flames produced during a combustion process to detect emissions in the ultraviolet (UV) light spectrum. Generally, wavelengths of 1,850-2,600 angstroms are monitored with ultraviolet detection tubes which operate on the principle of the Geiger Muller tube.
Most UV detector tubes consist of a helium-filled, glass enclosure containing parallel tungsten electrodes. A high voltage DC source is applied across the tungsten electrodes. Whenever the tungsten electrode is struck by an ultraviolet photon with an energy level in excess of the photoelectric work function of tungsten, the electrode emits an electron.
Each photon produces the emission of one electron that is amplified to an electronically detectable level by helium in the UV tube as follows: The high voltage differential between the electrodes accelerates the freed electron to the opposite electrode; the freed electron collides with helium molecules in the process of acceleration. With each collision, an additional electron is freed and this electron is also accelerated toward the opposite electrode, in turn striking additional molecules of helium and producing a chain reaction referred to as a Townsend Avalanche. As a result, the UV tube alternately conducts and quenches as ultraviolet photons strike the tungsten electrodes. In this way, a pulse train is generated having a repetition rate proportional to the emissions in the ultraviolet spectrum from the monitored flame.
As tubes age, they exhibit a gradually increasing tendency to produce an output signal in the absence of ultraviolet radiation. Such false output distorts the results obtained for the process monitored. Thus, unreliable results are produced, often defeating the purpose for which monitoring was intended, and in some cases exacerbating the problems attendant with controlling the process.
An aging ultraviolet tube eventually fails. If a critical combustion process is being monitored for flame activity, appropriate flame control may not be provided. Accordingly, the combustion process can either run away or become extinguished, producing dangerous and damaging results.